In conventional interferometric radio direction finding systems, the phase difference between signals received by the two sensors is measured to determine the angle of arrival of the signal since, for a fixed separation distance between the two sensors, the phase difference is directly related to the angle to be measured. However, interferometric techniques to measure the angle of arrival of an RF signal have posed problems in implementation of the system and accuracy of the measurement. Traditionally, a long base line has been preferred to reduce the effect of noise and instrument error. However, a long baseline is not practical in mobile site applications, such as airplanes, where the physical parameters of the measurement site are severly limited.
Also, conventional interferometers even those having a long baseline are restricted in the use of the long baseline because the accuracy is greatly reduced by decorrelation of the signals at the two interferometric sensors due principally to the differences of scattering and multi-path propagation effects at the two sensors. In addition, conventional interferometers are also particularly frequency sensitive and suffer from limited resolution and phase ambiguity problems.
An alternate approach to determining the angle of arrival of a RF signal is to measure the time of arrival difference of the signal between the two sensors. This can be accomplished for any baseline length so long as the difference in arrival times can be accurately measured since, for a fixed baseline, the delay is a known function of the angle to be measured. The present invention has incorporated the differential time-delay concept by an adaptive loop, without actually measuring the delay.
Art known to the applicant in the area of interference cancellation systems for eliminating interference in radio receivers which employ an adaptive loop is found in U.S. Pat. No. 3,669,444 to the present co-inventor, R. N. Ghose. Since it is highly desirable to be able to simultaneously transmit and receive on a single frequency from adjacent antennas, it was necessary to develop a system which could cancel out the unwanted transmitter signal from the received signal and, hence, allow full duplex radio transmissions on a single frequency. The Ghose invention when connected to the radio receiver antenna will sense the interfering signal and generate a signal which is the negative compliment of the interfering signal. The negative compliment signal is added in the radio receiver antenna circuit to cancel out the interfering signal.
To assure that only the transmitter reference signal is cancelled out and the correction loop is not disturbed by stray signals from other sources which would cause the correction loop to operate incorrectly, the inventor created a stage which takes a sample of the signals delivered by the transmitter system to the receiver antenna. Then another stage exists to pass into the summing coupler the required cancellation signal obtained from the reference sample following appropriate adjustment of its amplitude ratio and its electrical phase angle as time delay.
Art known to the applicant disclosing signal controllers can be found in U.S. Pat. No. 4,016,516 to W. Sauter, and U.S. Pat. No. 3,648,176 to D. Martin. The signal controller disclosed in the aforementioned Sauter Patent uses PIN diodes and is designed to be inserted into an RF transmission line to control the signal amplitude ratio and polarity by an external DC voltage control means. It is embodiments of this reflective signal controller which are primarily used as signal controllers in the present invention. U.S. Pat. No. 3,648,176 discloses a signal controller to cancel out unwanted signals by employing an RF potientiometer directly coupled to a drive motor and a velocity generator whereby corrections in potientiometer may be made automatically by driving the motor in a feedback control loop.
Art known to the applicant disclosing the use of interferometric techniques is U.S. Pat. No. 3,716,863 to R. N. Ghose and W. A. Sauter for an instrument landing error correcting system. The Ghose et al invention is an automatic correcting system for Instrument Landing Systems (ILS) used as a landing aid for aircraft. It involves reduction or correction of the 90 or 150 Hz moduation output for the ILS receiver by radiating a correcting signal which when detected by the ILS receiver, nulls the dominant modulation error resulting from the building-reflected signal.
The foregoing art references do not give any directional information with respect to the signals received, therefore, the present invention is highly distinguishable from these references.